banorkin on mon 24 jan 00
Crystalline glazes are not as hard to control precicely as some people
think. The main difficulty in obtaining results is that crystalline glazes
are in general fluid when melted, and when they are fluid the chemical
composition of the glaze coat is constantly changing, the resultant
composition once the glaze has stopped flowing may or may not support the
growth of crystals. In my initial trials, i monitored the firing cycle
manually, and i noticed that when I was able to reproduce a cycle I noticed
that what formed in the glaze as well as the way what ran off of the vase
looked the same. When I failed, what formed in the glaze as well as how
what ran off of the vase looked different. In order to run the same cycle
exactly precicely, I decided to set up a temperature controller to run the
kiln electronically that did not rely on the use of cones. I ran into
several problems, the solutions for which are relatively simple.
The difficulty with programmable digital temperature controllers has to do
with their stability and getting an accurate and repeatable reading from a
temperature sensor. With my set up, until I resolved these problems the
controller seemed not to work at all. I purchased my temperature
controller from Research Inc. in minneapolis minnesota. They no longer make
temperature controllers, however, I believe that the problems that I had to
resolve for this piece of equiptment would hold true for other pieces of
equiptment made by other manufacturers.
The stability of the controller refers to the error produced in the process
temperature as the ambient temperature changes around the controller. For
my controller the error in the process worked out to .275 degrees C for
every change in one degree C about the controller. This does not sound like
very much. However if the controller is in an uncontrolled environment the
changes in temperature around it can be enormous. For an error of ten
degrees C the process temperature would be elevated or depressed 2.75
Degrees C. Consider that in the final upward advance of temperature that
the controller is commanding the temperature to change 2 degrees C per
minute. The controller will spend plus or minus 44 seconds at the top
temperature while the glaze is fluid. Imagine if you were waiting for a
cone to bend, and once it was in the proper position that you waited another
44 seconds! The results would probably be slightly different. with
crystalline glazes the results would be a lot different. I have two
solutions to this problem. I work at home, and in the back bedroom of my
house where I keep the temperature controller i put an air conditioner in
the window during the summer and set the thermostat to keep the room at 70
degrees F. In the winter I set the thermostat to the house at 70 degrees
and left the
airconditioner on all year long. Essentially, what I have made when i
close the door to the room is an environmental chamber that has a constant
temperature all year long. The room is 10 by 10 by 8 feet tall and the
airconditioner is a 5000 btu model from Sears. In this environment the
controller never introduces an error in the process temperature. The second
solution was worked out by thurmond gardner and his assistant who both built
scientific equiptment for the science departments of Old Dominion University
in Norfolk, Va. In a used refrigerator that I purchased from the salvation
army, they installed a dual stage thermostat that took cold air from the
freezer compartment and introduced it into the lower refrigerator section
when needed. In the bottom of the refrigerator they installed in what
looked like a country mail box three conical porcelain spring heaters. The
mail box was open at both ends with a small fan at one end. When heat was
needed the fan and heaters came on together. at the other end of the
mailbox heating system was a small fan that directed air upwards in the
refrigerator and remained on all the time to circulate the hot and cold air.
The refrigerator had a non condensing atmosphere so no moisture formed on
the electronic equiptment. The door to the refrigerator had a plexiglass
window installed almost as large as the entire door so I could see the
equiptment without opening the door. There were actually two panes of
plexiglass, one on the inside of the door and one on the outside of the
door. Through them was an opening that could be opened or closed that i
could reach through to program the controller. So they made an
environmental chamber out of a refrigerator very cheaply. It kept the
temperature to within 2 degrees F. The thermostat was relatively cheap and
was purchased from W W Grainger and Company.
Cold junction compensation is another factor that must be attended to.
Here, the electronics of the controller will measure the temperature of the
cold junction of the thermocouple and adjust for changes in ambient
temperature around it which can cause an error in the thermocouple reading.
My controller has cold junction compensation measured from the back board of
the controller. This would encourage me to place the controller as close as
possible to the thermocouple as I could. If I did this, I would have a
terrible stability error to deal with. The solution is to us the standard
industrial set up for Platinum thermocouples. The standard solution if an
aluminum dust cover with a high purity closed inded protector tube cemented
into one side of it and the lead wire from the controller to the
thermocouple entering the other side. In electrically noisy environments,
the lead wire has to be shielded with a flexible stainless steel overbraid
which will eliminate noise from power controllers from being picked up
completely. Over the overbraid, I had my thermocouple maker add a plastic
jacket so that at the point where the extension wire entered the dust cover
the rubber compression fitting could make a positive seal to prevent any
dust from entering the tube. The copper extension wire itself must be
compensated for PLartinum thermocouple wire. Inside the dust cover the
connections between the platinum wires and the lead wires are made through
an uncompensated porcelain junction block. The terminals are four in number
and are generally steel skrews into brass. When making connections at the
cold junction Metals dissimilar from the compensated extension wire and
Platinum wires can introduce an error in the reading except under the
condition that all of the connections are at the same temperature. so the
porcelain block very nicely keeps all of the connections at the same
temperature and there is no error at all. In noisy environments the hot
junction of the thermocouple, the end that senses temperature in the over,
must be grounded. At the junction block it is connected to the stainless
steel overbraided shield of the extension wire. I hade my assembly
manufactured by CGS Thermodynamics, Inc. It was set up by Fred Leach who
seems to know everything there is to know about setting up sensors. I have
my thermocouple manufactured with the extension wire in place so that I
never have to open the dust cover. If silica gets into the tube, it can
deposit on the platinum elements and a compound called platinum disiliside
will be formed. The introduction of another compound into the thermoucouple
wire essentially creates a new and unknown alloy that will ruin the
temperature readings from the sensor forever. When I look at some hobby
controllers even very good ones i have noticed that the extension wire does
not have a stainless steel overbraid. I assume that this extension wire has
an internal shield that is a plastic impregnated with metal. In the Omega
Engineering Temperature Measurement catalogue(the seven eleven of
temperature control supplies) it states that compensated wire shielded like
this REDUCES PICKUP from the power controller. So there will always be a
variable error depending on the output of the controller introduced into the
reading of the thermocouple. I have noticed that on some hobby contollers
and even on what are supposed to be
some very good controllers that the connection between the extension wire
and the platinum sensor is made through
what is called a compensated quick disconnect plug. Compensated is a little
misleading. It implies that the connections are not subject to a cold
junction error.and if the connections are all made with compensated
materials it would not matter what the temperature about the connections of
the plug was, there would be no error.
In the Omega Engineering catalogue I noticed that the color of the skrews
were a different color than the compensated copper male and female lugs. So
I called the manufacturer to ask about it. He replied that it did not
matter that the skrews were not made from compensated copper that the error
cancelled itself out for the most part but
that there was a very small error introduced into the reading. So I asked
him if the error might be more than say thirty millionths of a volt. His
reply was that oh it was larger than that. I thanked him and hung up. It
was a leading question, because if you look into the tables for the voltage
generated for every degree of temperature you will find that for a type R
thermocouple, that at the temperatures around cone ten that four to sever
millionths of a volt represents one degree centigrade. so the plug will
introduce a variable error in the reading of four to seven degrees C if the
error is thiry millionths of a volt or less! In addition, the high purity
protector tube is inset into one side of the plug. So at high temperatures
one side of the plug is slightly higher in temperature than the other and
this introduces the error that I just spoke about. the theory that if all
connections are at the same temperature uncompensated metals may be used to
make the connections no longer holds true. In addition, the High purity
protector tube is inset into one side of the plug and this does not seal it
off from the environment. For all of the above reasons, I believe this is
why people have trouble getting repeatable results with some controllers
when they try to make crystals. In addition to all of this, I selected a
controller that allowed it to count out temperature 1/10 of a degree C at a
time. This allows me to have the controller walk the temperature right up
to the same temperature each time to within one tenth of a degree. If the
controller counts temperature in incriments of one degree then it is only
repeatable to within one degree! This will produce crystalling results
every time, but they will vary somewhat from firing to firing. Once the
controller is stabalized and the temperature sensor is proberly set up
repeatable results can be obtained from firing to firing like clockwork.
However the design of the oven must be such to allow for such precice
control and there are other variables associated with the preparation of the
glaze itself that must be taken into account for this scheme to work.
As far as the kiln is concerned, it must have a static atmosphere. This
means, no updrafts, downdrafts or cross drafts. My ovens are top loading
kilns that I built myself. The barrel of the kiln is two bricks thick. The
inner element bearing face is made from 2500 degree F insulating brick and
the outer face is made from 2000 degree F insulating brick. The outer
bricks do not but up against one another. Inbetween them is a v shaped
space that i fill with 2000 degree insulating castable. The floor of the
kiln is made from 2500 degree F brick and is made like a kiln lid that
extends to the outer dimension of the barrel of the kiln. It sits upon a
layer of 1/4 inch 2300 degree fiber board (to cut out up drafts) and that
sits on a layer of 2500 degree insulating brick. The oven itself sits on
cinderblocks with the holes in the block open to the atmosphere to allow
circulation under the kiln. The barrel of the kiln is held togehter by
stainless steel hose clamps. I buy the flat kind instead of the coiled
clamps that are impossible and dangerous to work with. My ovens are a nine
sided kiln that is 18 inches tall, and a ten sided kiln that is 13 and 1/2
inches tall. Each tier of bricks in the barrel is held together by one
continuous hose clamp(usually it takes two joined together to fit around the
oven) Both ovens have eight 1/2 lb. 16 gague kanthal A-1 heaters coiled to
an outer diameter of .40 inches. I purchase closed coils in one pound
lenghts from Duralite in riverton connecticut.
. The selection of inner face and outer face bricks was worked out on a
computer program at the refractory brick company where I buy bricks. I told
them what temperature i wanted the outer face to be at when the inside of
the kiln was at 2400 degrees F. I wanted the outside temperature to be
about 90 degrees F. They recommended that with a 2500 degree inner face I
should use an 1800 degree outer face. Since they were out of 1800 degree
brick, and i was anxious to build, I used the 2000 degree brick. The ovens
are extremely cheap to run. In each of the kilns I have drilled several
holes to allow the thermocouple to be inserted at different heights. the
diameter of the hole should fit the thermocouple tube snugly. I have to make
certain that when I am using one hole that the others are plugged up.
Otherwise this can result in a variable error in the temperature reading in
these ovens of 4 to 5 degrees C. The lid is made from 2500 degree
insulating brick. it has to only cover to the diameter where the inner and
outer face bricks meet. I cut the element trays and facet the innerface
bricks on a miniature table saw. Before I cut the brick, I wet them
thoroughly to cut down on dust and wear eye protection. These kilns are way
overpowered. so the heaters could be lighter that 1/2 lb. Actually, heaters
of an ohms length of between 16.2 and 27.5 work well and allow me to fire to
cone ten at a percentage output of 70 percent or less at the top temperature
of cone ten in four hours and fourty five minutes.
Mixing the glaze is as important as anything else. My glaze consists of
ferro frit 3110 250
300 mesh
silcosil 145
calcined zn
Denzox brand 165
Bentonite 15
Lithium carb.
granular or
powdered 1
Ferro Frit 3110 must be passed dry through a 200 mesh stainless steel sieve
with a metal frame to hold the screens calibration stable. I gently brush
the frit through the screen using a special brush designed for this.
. If I dont do this, I get variable results even with precice temperature
control and an oven that is set up properly. I mix my glazes in an Oster
blender. The first ingredient is Bentonite. For a double batch I add a
little less than two cups of water. I blend in the Bentonite first, working
the speed up to the highest. According to a process engineer at ferro corp,
Mr. Wizkowski
this flash blends the bentonite into water so that the platelets are
actually inbetween the molecules of water. Next I add ferro frit, then zinc
then sand, and metal oxides as coloring ingredients and then lithium. The
lithium creates a thixitropic suspension that never settles provided I do
not thin the glaze out with too much water. Mixed like this it does not
matter whether or not the glaze is brushed on vases or poured over vases,
the results are the same. I have never sprayed glazes, so I dont know if
this glaze works well when sprayed on. The glaze can be stored in air tight
plastic containers where it will not settle and no water forms above the
glaze, or it can be allowed to dry in a plastic container where it will
shrink away from the container as it drys forming a hard pellet that can be
reconstituted at a later time by breaking it up into small pieces and adding
it to water. The inside of the vase can be crystal glazed by cutting the
glaze with excess water and swirling it around the inside of the vase to
deposit a thin coating. The inside of the rim has to have a lot of extra
glaze built up. As it flows over the thin coating it crystal glazes the
inside without a lot of pooling of glaze in the bottom of the vase. If I
put too heavy a coating on the inside where the glaze pooled the bottom of
the vase will shear off from the top during the cooling cycle. I like my
vases to be glazed inside and out. And anymore, I never lose a vase to
excessive pooling.
>From a 92 year old
fith generation potter, Palin Thorley, I learned that I could line the
bottom of my oven with an inch and 1/2 layer of alumna. This would allow me
to fire my vases on insulating brick. When the glaze runs off the vase it
simply pools ontop of the alumna. I use product 661 tabular alumna 100 mesh
manufactured by alcoa aluminum. I purchace in minimum lots of one hundred
pounds.from Whittaker Clarke and Daniels. The mesh size is very important,
if it is too fine it will absorb the glaze instead of allowing the glaze to
pool. Every so often, more alumna must be added to make up for some that is
absorbed by the run off.
The vases are fired on 2600 degree insulating brick manufactured by BNZ
Materials inc in Zeleonople pennsylvenia. The consistency of this brick is
gorgeous and it will not melt out from under the vases. The pedestals need
not be very thick to work. About 1/2 inch. I cut the brick with a silicon
carbide tipped hack saw blade. they last for years. regular hack saw blades
last a very short time before they get too dull to even cut the brick. I
round off the brick on a solid slab of cinderblock. I wear cheap rubber
dishwashing gloves when I make the vase pedestals. Since the pedestals are
going to be placed on a bed of alumna they do not have to be cut so
perfectly that the top and bottom are parallell.During the cooling cycle the
brick usually shears off on its own or a fracture between the brick and the
vase occurs which with the gentle tapping of the plastic handle of a
skrewdriver seperates the brick from the vase. Then there is a minimum of
grinding necessary. Some melts are harder to grind than others. Copper is
the easiest, Manganese is the worst. To prevent the glaze that runs off of
the vases from touching the barrel of the kiln, I line it with slivers of
BNZ brick, several layers deep so that if the glaze touches that it can be
removed and easily replaced. When I fit the bricks to the base of the vase
I try to indent the brick slightly, so that as the porcelain vase shrinks
the brick does not extend beyond the outer diameter of the base of the vase.
if that happens then the glaze can pool and become thick and uneven about
the base. The base of the vase has a WIDE ring to allow the vase to shrink
over the pedestal and allow for indenting the vase pedestal.
To clean up the bottom of the vases, I use an electric polisher with central
water feed model AEP-458 manufactured by Alpha Professional Tools. It is
actually a watercooled angle grinder that uses a 7 inch diamond disk as the
grinding surface. There are two types of pads that will hold the disk in
place, I use the pad that has a foam rubber layer to support the diamond pad
instead of the rigid pad. I use 60 and 70 grit diamond disks.On smaller
diameter vases I can do about 400 vases per disk before it becomes too dull
to use. For larger based vases each disk will easily grind off only about
40 and then a lot of smaller vases. I mounted the grinder
upsided down in a plastic closet storeage box. I cut a hole through the
plastic lid over the grinding disk, this arrangement makes it hard to slip
against the grinding disk and provides a surface to lean my hands against
when grinding vase bottoms. After i have ground off the excess glaze from
the bottom of the vase, the vase can be centered over the wheel and ground
again
and the bottom comes out perfectly flat and smooth as glass.The grinder is
elevated above the floor of the container to keep it above water if the
drainage holes get plugged up somehow. Unlike dry grinders, it is very
forgiving if I touch it momentarily. I can actually hold my finger gently
on its spinning surface and experience no damage, nails on the the other
hand it takes off in
a flash.It took quite a bit of practice to learn how to use it, and several
disks that i thought were dull are still usable. The mist that it puts out
is loaded with fine powder, so good ventilation and respiratory protection
is essential. I grind my vases out doors. And for respiratory protection,
I use an air assisted hepa filtration system that employs a half mask. The
theory is that clean air is blown into the mask. so if a gap occurs in the
seal of the mask against my face, air exits the mask and dust cannot enter.
With regular cartrage hepa filter masks if dust is blown at the mask, it can
enter through the exhallation valve as
I breathe. The bottoms of my vases have wide rings to allow for shrinkage
over the insulating brick.
Using the methods described above it is really possible to see how small
changes in the glaze composition influence the results, or
what the effect of changing the firing cycle is or what the effect of
growing crystals at one temperature versus anothe actually is. In other
words, I can make meaningful experiments and reproduce results sometimes for
years at a time. Sometimes a manufacturer will discontinue a product and
then
I will lose a result.
The last items are my firing cycle and my clay body composition.
FIRING CYCLE
67 degrees C 1 second to get there
972 3 hr 8 min
992.7 2 min 59 sec
1042 9 min
1047.1 1 min
1057 2 min
1061.9 1 min
1102.1 9 min
1110.6 2 min
1127 4 min
1142.2 4 min
1153 2 min 57 sec
1166.8 4 min
1179.8 4 min
1192 4 min
1200.6 3 min
1202.1 4 min 16 sec
1223.8 4 min 30 sec
1235.1 4 min 31 sec
1249 5 min 47 sec
1256.9 3 min 9 sec
1262 2 min
1269.6 3 min
1277.1 3 min
1289.1 5 min
1305.1 7 min
1110 1 sec
1110 6 hours for overall crystals, for seperate
crystals back down to perhaps 4 1/2 hours-arrived at by trial and error.
0 1 second
I use all of these steps because i have a controller that is capable of up
to 52 segment programs. so I was able to curve fit my program.to a
successful cycle that I ran manually.
I have run a straight line program and it works just fine.too.
In the system that I have described above the random growth of crystals with
uniform distribution about the vase is possible every time simply by
adjusting the length of time crystals are grown to form aggregates. In
general I grow crystals at a constant temperature and there is a wide range
of temperature over which they can be grown. and at each level of
temperature, usually every 15 degrees C I notice a change in the crystal
structure obtained.
My clay body
I slip cast my work in order to have control over the clay.
grolleg powder..30parts by weight
tennessee
ball #10 ............ 15
silcosil
silica
200 mesh ..........20
monticello
g-200
feldspar ............. 35
I mix 13 gallons of water with 230 pounds of dry powder and 200 ml of sodium
silicate
I also use a manufactured clay body, Standard Clay body 257 for the potters
wheel.
but the consistency really differs from batch to batch.
I am writing all of this down in the hopes that these ideas will help a lot
of people to get started making crystals from the first firing and give them
the means to express their imaginations as to what they think crystals
should look like, the possible results seem to be infinite. I would like to
see high schools and colleges set up so that students can make this glaze
and work meaningfully, and most of all when I go to shows I would like to
see a lot more people out there with wonderful crystalline glazes.
This is not the only way to make crystalline glazes work, but I guarantee
you, if you try it, you will get results, and quickly. And you will be able
to repeat wonderful results.
good luck
Bevan Norkin
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